Apparatus and method for removing mercury from a gas

ABSTRACT

The present invention provides a method and apparatus for removing mercury from gases such as those discharged from roasters and other heat producing systems. In embodiments the method comprises reacting the mercury with dissolved molecular chlorine, and may also comprise reacting the mercury with mercuric chloride to yield mercurous chloride. The mercurous chloride may be removed by precipitation. There are also disclosed apparatuses for implementing the method.

BACKGROUND

Mercury emissions from coal-fired boilers and from other combustionprocesses, including municipal waste combustors, gold ore roasters, andhazardous waste incinerators have been studied to determine emissionsrates and effectiveness of control technologies. Factors that differbetween coal-fired boilers and ore roasters may include mercury contentof the feed and flue gas, flue gas sulfur dioxide concentration, andflue gas mercury speciation. Analyses of untreated flue gas fromcoal-fired boilers have detected mercury concentrations up to 25 ug/m³,with an average concentration of 15 ug/m³. Gold ores have measuredmercury concentrations from less than 1 mg/kg to 200 mg/kg for oredeposits in the western U.S. It is thus desirable, and in many countriesit may be legally required, to remove contaminating mercury from offgases before these are released to the environment.

Studies of treatment technologies for mercury removal from coal-firedboiler emissions have shown that speciation of mercury may impactmercury removals. Mercury species in combustion gases may includeelemental mercury vapor (Hg⁰), oxidized mercury vapor (Hg⁺²) and mercuryassociated with particulate matter (Hg^(P)). Elemental mercury may be adominant species in roaster flue gas.

It is thought that the mercury in off gases from ore roasters and thelike may be present as water-insoluble elemental mercury (Hg) andwater-soluble ionized mercury (Hg²⁺). Although ionized mercury, which iseasily absorbed by water, can be removed by a desulfurization absorptiontower or the like, elemental mercury, which has a very low solubility inwater, may not be absorbed by a desulfurization absorption tower, andmay be discharged as elemental mercury vapor without being absorbed.

Conventionally, two major approaches have been proposed for the removalof mercury from gases: an activated carbon adsorption method and amercuric chloride absorption method in which elemental mercury isconverted to mercurous chloride by the reaction with mercuric chloride.However in the foregoing conventional mercuric chloride method, when themercury having been oxidized once is trapped by a downstream scrubber,it may be reduced by the action of SO₂ coexisting in the flue gas, andmay be volatized again. The effectiveness of the conventional mercuryremoval process may be limited by the reaction rate of the mercuricchloride with the elemental mercury.

A variety of methods and apparatuses for the removal of mercury fromcontaminated gases are well known in the prior art.

U.S. Pat. No. 5,409,522, Apr. 25, 1995 to Durham et al. discloses anapparatus and process for removing particular material andmercury-containing compounds from a gas stream by regenerating thesorbent used to recover the mercury.

U.S. Pat. No. 6,920,329, Nov. 1, 2005 to Sellakumar discloses anapparatus and process for converting mercury to mercuric chloridecomprising contacting the mercury with a solution of a chloride salt andheating the solution/

FIELD

The present invention relates to a method and apparatus for removal andrecovery of mercury from gases.

SUMMARY

In an embodiment there is disclosed a method for removing mercury from agas, the method comprising reacting the mercury with dissolved chlorinegas to form mercuric chloride.

In alternative embodiments the chlorine gas and the formed mercuricchloride are in aqueous solution.

In alternative embodiments the solution and the gas may form agas/liquid interface and the reacting may further comprise forming a2-dimensional mercury vapour at said gas/liquid interface.

In alternative embodiments the reacting may occur at a temperature ofless than about 65° C. or at temperatures of less than about 40° C.

In alternative embodiments the method further comprises the step ofreacting any unreacted mercury with the mercuric chloride to formmercurous chloride and may comprise the step of removing the mercurouschloride from the solution.

In alternative embodiments the chlorine solution is at a pH of betweenabout 6.5 and about 7.5 and in embodiments the gas may be an off gas.

In alternative embodiments there is disclosed an apparatus for removingmercury from a gas, wherein the improvement comprises a first mercuryscrubber for reacting the mercury with a chlorine solution.

In alternative embodiments the apparatus may comprise a chlorine feedsystem for regenerating the chlorine solution.

In alternative embodiments the improvement further comprises a secondmercury scrubber and a circulation system suitable to substantiallycontinuously recirculate the chlorine solution between the two saidscrubbers.

In alternative embodiments the apparatus further comprises a harvesterfor harvesting precipitated mercurous chloride generated in thescrubber.

In embodiments there is disclosed a method for removing mercury from agas wherein the improvement comprises reacting the mercury with asolution of chlorine gas to form mercuric chloride in solution.

In alternative embodiments the method comprises reacting the unreactedmercury with the mercuric chloride to form mercurous chloride.

In alternative embodiments the method further comprises the step ofremoving the mercurous chloride from the solution.

In alternative embodiments the solution and gas form a gas/liquidinterface and said reacting further comprises forming a 2-dimensionalmercury vapour at said gas/liquid interface.

In alternative embodiments the reaction of mercury with said solution ofchlorine gas occurs in a first scrubber and said reaction of mercurywith mercuric chloride occurs in a second scrubber.

In alternative embodiments the reacting may occur at a temperature ofless than about 65° C. or at temperatures of less than about 40° C.

In alternative embodiments the reaction between the chlorine and themercury occurs under an ORP of between about 500 mV and about 2000 mV.

In an embodiment there is disclosed a method for removing mercury from agas wherein the improvement comprises reacting the mercury with chlorinegas to form HgCl₂.

In an alternative embodiment there is disclosed a method for removingmercury from a gas wherein the improvement comprises reacting themercury with chlorine gas to form HgCl₂ and wherein the chlorine gas isin aqueous solution.

In an alternative embodiment the method further comprises reacting themercury with the HgCl₂ to form Hg₂Cl₂.

In an alternative embodiment there is disclosed a method for removingmercury from a gas wherein the improvement comprises reacting themercury with dissolved chlorine gas.

In an alternative embodiment there is disclosed a method for reactingmercury with chlorine gas wherein the improvement comprises reacting themercury with the chlorine gas at a gas liquid interface.

In an alternative embodiment there is disclosed mercury scrubber whereinthe improvement comprises a reactor for reacting the mercury with asolution of chlorine gas.

In an alternative embodiment there is disclosed an apparatus forremoving mercury from a gas wherein the improvement comprises a mercuryscrubber for reacting the mercury with a solution of chlorine gas.

In an alternative embodiment there is disclosed an apparatus accordingto other embodiments wherein the apparatus further comprises a secondmercury scrubber for reacting the mercury with a solution of HgCl₂ andgenerating Hg₂Cl₂.

The apparatus according other embodiments wherein apparatus comprises aharvester to remove precipitated Hg₂Cl₂ from the solution.

In a further embodiment there is disclosed a mercury scrubber whereinthe improvement comprises a reactor for reacting the gas to be treatedwith a chlorine solution and wherein a further improvement comprisesprecipitating mercurous chloride.

In embodiments the mercury may be elemental mercury.

Features and advantages of the subject matter hereof will become moreapparent in light of the following detailed description of selectedembodiments, as illustrated in the accompanying figures. As will berealized, the subject matter disclosed and claimed is capable ofmodifications in various respects, all without departing from the scopeof the subject matter hereof. Accordingly, the drawings and thedescription are to be regarded as illustrative in nature, and not asrestrictive.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a flow diagram showing the steps of an embodiment.

FIG. 2 is a flow diagram of an embodiment showing monitoring and controlfunctions at each stage.

FIG. 3. is a schematic diagram of mercury scrubbers of an embodiment.

DETAILED DESCRIPTION OF EMBODIMENTS Terms

In this disclosure a mercury scrubber means any device for removingmercury, from a gas.

In this disclosure the terms “off gas” and “flue gas” mean gas releasedby a process, which may be or may comprise a heating or combustion stepand in embodiments may include gases generated by ore roastingprocesses, coal and oil fired boilers and hazardous waste incinerators.

In this disclosure “mercury” includes elemental mercury.

In this disclosure the term “soda ash” has its normal meaning and meanssodium carbonate, which may be in solution or solid, as the contextrequires. Those skilled in the art will readily store and handle sodaash and will prepare and use suitably concentrated stock solutions.

In this disclosure the term “scrubber solution” means solution to becontacted with gases to be treated to remove mercury therefrom and inembodiments may contain dissolved chlorine, may contain dissolvedchloride ions, and may contain a pH adjusting component which may be ormay comprise soda ash or equivalents.

In alternative embodiments an ore referred to in this disclosure may beany metal ore including but not limited to ores of metals, which may beheavy metals or precious metals, and may include ores of gold, silver,nickel, iron, zinc, cobalt, manganese, molybdenum, vanadium, tungsten,lead, tin, tantalum, cadmium, arsenic, aluminium, beryllium, bismuth andcopper.

In this disclosure the term “gas to be treated” means any gas to betreated to remove mercury, and may be an off gas or flue gas.

In this disclosure the terms “harvester” “precipitator” “precipitationdevice” and the like all refer to any apparatus or device wherebyundissolved chemicals or materials may be removed from a solution orliquid by precipitation. In particular embodiments such apparatuses andmethods may be used to remove Hg₂Cl₂ (mercurous chloride or calomel)from a solution.

In embodiments gases treated by the methods and apparatuses ofembodiments may, after treatment, comprise less than about 10 μg/m³,less than about 9 μg/m³, less than about 8 μg/m³, less than about 7μg/m³, less than about 6 μg/m³, less than about 5 μg/m³, less than about4 μg/m³, less than about 3 μg/m³, less than about 2 μg/m³, less thanabout 1 μg/m³, less than about 0.9 μg/m³, less than about 0.8 μg/m³,less than about 0.7 μg/m³, less than about 0.6 μg/m³, less than about0.5 μg/m³, less than about 0.4 μg/m³, less than about 0.3 μg/m³, lessthan about 0.2 μg/m³, less than about 0.1 μg/m³, or less than about 0.05μg/m³ of mercury. In particular embodiments the treated gases maycontain less than about 6 μg/m³of mercury.

In embodiments particular steps may be repeated, and the apparatuses andmethods described may be combined with any other methods and apparatusesfor treatment of gases in ways readily apparent to those skilled in theart. Specific concentrations and handling of reagents will be readilyunderstood and modified by those skilled in the art to suit particularrequirements.

FIRST EMBODIMENT

A first embodiment is described with general reference to FIGS. 1, 2 and3 and comprises a method and apparatus for removing mercury from a gas,which may be an off gas and may be generated by ore roasting. In anembodiment the ore may be gold ore. The embodiment is furtherillustrated in detail in an example presented below.

The overall process is generally designated 10 and may comprise a numberof steps, each of which may be associated with a designated portion ofan apparatus as illustrated more particularly in FIGS. 1 and 2.

In a first step designated 100 a gas to be treated, which may be an offgas may be produced by ore roasting and may be at high temperature. Thegas to be treated may be cooled (110) using water sprays, heatexchangers, refrigeration systems or any other suitable means, and suchcooling may occur in a quench tower and in particular embodiments mayresult in a gas with a temperature below about 65° C., 60° C., 55° C.,50° C., 45° C., 40° C., 35° C., 30° C. or lower than about 30C, or aboveone of the foregoing temperatures. Following passage through the quenchtower particulates may be removed (120) in a dust scrubber, wet scrubberor other suitable device to remove particulate matter. Suitablealternative devices may include cyclones, filters, dust separators, andthe like and particulates may also be removed by forming a water slurrywhich may then be further processed to remove gold or other desirableconstituents. In embodiments the gas leaving the dust scrubber may havea further reduced temperature and may be cooler than about 50° C., 45°C., 40° C., 35° C., 30° C., 25° C., 20° C. or less.

Sulphur dioxide may be removed (130) in a sulphur dioxide scrubber. Thismay be achieved by water or alkali scrubbing; and may be accomplishedusing a packed tower scrubber using soda ash addition to control pH. Itmay be important to remove SO₂ from the gas stream prior to oxidation ofthe mercury, since SO₂ may act as a reducing agent and will interferewith the proper operation of the mercury scrubber. After removal ofsulphur dioxide the gas pressure may be increased, for instance by adraft fan, and the pressure may be raised to greater than about 2 poundsper square inch (psi), 2.5 psi, 3.0 psi, 3.5 psi, 4.0 psi, or more.Within the SO₂ removal stage 120 both pH and temperature may beregulated

The removal of mercury may be achieved by reacting the mercury withdissolved chlorine gas in a first scrubbing step 140. As will be seenfrom FIG. 3 this may be accomplished using a first mercury scrubber 300also referred to as mercury scrubber (I) or mercury scrubber 1, to formmercuric chloride (HgCl₂). In mercury scrubber (I) mercury may beabsorbed or adsorbed onto a solution/gas interface and two reactions mayoccur:

Hg⁰+Cl₂->Hg²⁺+2Cl⁻  A.

and

Hg⁰+Hg²⁺+2Cl⁻->Hg₂Cl₂;   B.

the Hg²⁺ ions partition into the scrubbing solution and Hg₂Cl₂ (calomelor mercurous chloride) precipitate may form.

The chlorine solution may also contain dilute soda ash to control the pHand may further contain mercuric chloride for reacting with mercury ionsthat may be present in the gas or may be generated by the reaction ofelemental mercury in the gas with molecular chlorine.

Any remaining unreacted mercury may be removed in a second mercuryscrubbing step 150 in mercury scrubber (II), also referred to as mercuryscrubber 2, by reaction with dissolved mercuric chloride, or dissolvedmercuric chloride and chlorine gas. Again the pH of the solution may beadjusted by the addition of soda ash.

FIG. 3 is a schematic diagram of the structure and relationship of thetwo mercury scrubbers of the first embodiment. The assembly comprisingthe two scrubbers is generally designated 800. Scrubber (I) 300, andscrubber (II) 400 may be of similar construction. Each of the scrubbersis fed with seepage or fresh water by a water supply 260.

Scrubber (I) comprises a containing wall 305, a demister spray 310, ademister screen 320, a packed bed 330, and a mercury collection assembly350. A number of access points 340 are provided. Soda ash is fed intoscrubber (I) 300 through a soda ash feed 670 as necessary to maintainthe desired pH, and chlorine gas is injected into the circuit (550) inresponse to monitoring by an ORP probe and liquid mercury is harvestedin mercury collection assembly 350. Scrubber (II) comprises a containingwall 405, a demister 410 spray, a demister screen 420, a packed bed 430,and a mercury collection assembly 450. A number of access points 440 areprovided. Soda ash solution is injected into scrubber (II) 400 throughsoda ash feed 660 as necessary to maintain the desired pH. A number ofprobe insertion points 370 are provided for probe insertion intoscrubber (I) and a number of probe insertion points 470 are provided inscrubber (II) and may be used to monitor pH, temperature, ORP and otherparameters as desired.

Gas to be treated is delivered to scrubber (I) 300 by ducting 200, andfollowing passage through the scrubber is conveyed to scrubber (II) 400through ducting 230. After passage through scrubber (II) 400 the treatedgas is passed on to a tails gas scrubber in ducting 250.

Chlorine solution is fed to the scrubbers through an ORP (oxidationreduction potential) circuit. The ORP circuit comprises a line 500leading accumulated scrubber solution from scrubber (I) to a pump 726,and thence to a line 510 carrying solution from the pump. Line 510splits into a line 530 and a line 520. Line 530 flows to an injectionpoint 460 in scrubber (II) 400 for mixing the scrubber solution with theoff gas. Line 520 feeds into the lower portion of scrubber (II) at aninfeed 525, and it is at this point in the circuit that the compositionof the recirculating scrubber solution is adjusted by the addition ofchlorine as necessary at an injection point 550. This feature is alsoreferred to as a chlorine regeneration system, or a scrubber solutionregeneration system, and indicates or may indicate a system or systemswhereby the scrubber solution or solutions may be continuouslyregenerated to permit continued recirculation of scrubber solutionwithin or between the two scrubbers. Scrubber solution from scrubber(II) 400 is led off through a line 600, leading to a pump 710 whichfeeds into line 620 to convey the scrubber solution to a heat exchanger730, and into a scrubber solution bleed line 630. From heat exchanger730 line 620 leads recycled scrubber solution to an injection point 360in scrubber (I) 300. A cooling water input 630 and output 640 areprovided for the heat exchanger.

Mercury collection assemblies 350 and 450 serve to collect any liquidmercury accumulating in the base of the scrubbers. They comprise twovalves 900, 910, and a mercury collector vessel 920. In operation valve900 is normally open and valve 910 closed, mercury drains through pipes930 and collects in vessel 920. When desired valve 900 is closed andvalve 910 is opened to allow flow through outlet pipe 940 and themercury is collected from the vessel 920. Then valve 910 can be closedagain, and valve 900 reopened and collection continued. As indicatedabove, where mercury is absorbed or adsorbed or otherwise contacts asolution containing mercuric chloride or other Hg²⁺ salts, the reactionHg⁰+Hg²⁺->Hg₂Cl₂ is believed to occur resulting in the precipitation ofHg₂Cl₂ (Calomel).

The scrubber solutions containing dissolved chlorine and/or mercuricchloride may be recycled between the scrubbers 300 and 400 andregenerated as necessary to continue the scrubbing process and inembodiments this recycling and regenerating may be achievedsubstantially continuously. As desired or as necessary to drive thereactions, precipitating mercurous chloride may be removed from thesolution; and mercuric chloride may be regenerated as necessary to runthe process.

In particular embodiments the scrubber solution and the gas to betreated may contact each other at a gas/liquid interface and thereaction may comprise forming a 2-dimensional mercury vapour at thegas/liquid interface. The mercuric chloride (HgCl₂) may then dissolveinto the solution and may further react with mercury in the gas to formmercurous chloride (calomel, Hg₂Cl₂) which can be removed byprecipitation.

As will be seen from FIG. 2, during passage through the first mercuryremoval stage (in mercury scrubber (I)) (140) pH, ORP and temperaturemay be regulated and during passage through the second mercury scrubberstage (150) pH may be regulated.

In embodiments the reaction between the mercury and chlorine may becarried out at a temperature of less than about 100° C., 95° C., 90° C.,85° C., 80° C., 75° C., 70° C., 65° C., 60° C., 55° C., 50° C., 45° C.,40° C., 35° C., 30° C. or lower. The reaction may occur in the absenceof an added catalyst and in particular embodiments may be conducted at atemperature of less than about 65° C. It is believed that inembodiments, while the reaction may still occur at detectable levels attemperatures substantially above 65C, the efficiency or effectiveness ofthe reaction may decrease at higher temperatures.

In embodiments the pH of the chlorine solution in mercury scrubber(I)may be optimised to promote the reaction between elemental mercury anddissolved chlorine gas. The chlorine solution may also contain mercuricchloride and may contain a pH adjusting agent such as soda ash. Inembodiments the pH of the chlorine solution in mercury scrubber (I) maybe between about 6 and 8, and may be between about 6.5 and 7.5 and maybe between about 6.7 and 7.2 and may be between about 6.8 and 7.0 and inparticular embodiments may be about 6.9. Suitable adjustments totemperature, pressure, pH and other parameters to optimise the reactionfor desired purposes will be readily recognised and made by thoseskilled in the art.

In embodiments the ORP of the scrubber solution in mercury scrubber (I)may be maintained at above about 600 mV, above about 700 mV, above about800 mV, above about 900 mV, about 1000 mV, above about 1100 mV, aboveabout 1200 mV or higher and in particular embodiments may be maintainedat at least approximately 900 mV. In embodiments the reaction occurs atan ORP between about 500 mV and about 2000 mV. In embodiments mercuricchloride levels may be maintained at levels of between about 500 ppm to2000 ppm, and in embodiments may be below about 500 ppm, may be betweenabout 500 ppm and about 1000 ppm, between about 1000 ppm and about 1500ppm, between about 1500 ppm and about 2000 ppm, between about 2000 ppmand about 2500 ppm or may be above about 2500 ppm.

In embodiments the pH of the mercuric chloride solution in mercuryscrubber (II) may be between about 7 and about 9, may be between about7.5 and 8.5 and may be about 8 and in embodiments the solution may alsocontain dissolved chlorine and may contain a pH adjusting agents whichmay be or may include soda ash.

It will be appreciated that the reaction conditions in mercury scrubber(II) and mercury scrubber (II) may be similar, except that by the timethe gas reaches mercury scrubber (II) the bulk of the mercury in the gaswill or may already have reacted with chlorine and/or mercury chloridein mercury scrubber (I). Thus the conditions in scrubber (I) areadjusted to favour Reaction A, between chlorine and mercury, and inmercury scrubber (II) they are adjusted to more directly favour thereaction between mercury and mercuric chloride to form mercurouschloride. Those skilled in the art, with the guidance provided herein,will readily adjust the conditions in such scrubbers to promote thedesired reactions.

Following the mercury removal step or steps, the processed gas to betreated may be further polished (160) in a tails gas scrubber whichserves as a backup cleaning device to remove any contaminants not caughtby the previous steps. It may contain plastic packing rather thantitanium packing. Operationally, it will act and serve as a “buffer” toabsorb soluble gaseous components not fully captured in the mercuryscrubbers. Soda ash (Sodium Carbonate) may be added as necessary for pHcontrol. Following treatment the gas may be released to the atmosphere(170).

In particular embodiments, after processing using the methods andapparatuses disclosed herein the treated gas may comprise substantiallyreduced levels of mercury. In embodiments the treated gas may containless than about 10 ug/m³ of mercury, or less than about 9 ug/m³ ofmercury, or less than about 7 ug/m³ of mercury, or less than about 8ug/m³ of mercury or less than about 6 ug/m³ of mercury, or less thanabout 5 ug/m³ of mercury or less than about 4 ug/m³ of mercury, or lessthan about 3 ug/m³ of mercury or less than about 2 ug/m³ of mercury, orless than about 1 ug/m³ of mercury or less.

In embodiments, at any given gaseous concentration of mercury, the rateat which mercury is removed from the gas may be dependent on theconcentration of mercury on the adsorbing or absorbing surface. This maydepend on: the surface area available for adsorption by the mercury, thetemperature of the liquid solution interface, and the rate at whichmercury is oxidized or reacts with the chlorine and/or mercuric chloridesolution and is removed from the surface. The flow rates andcompositions of the chlorine, and mercuric chloride solutions may becontrolled by feedback from one or more ORP (oxidation-reductionpotential) probes provided in the relevant feed lines as will be readilyunderstood by those skilled in the art. Similarly pH controllers may beused to control the flow of soda ash or any suitable equivalent, inorder to maintain the pH of a solution at or near its desired value.

In embodiments the chlorine solution and/or the mercuric chloridesolution may be regenerated and/or recycled within or between one orboth of mercury scrubbers (I) and (II) and in embodiments the scrubbersolution may be recycled in a continuous flow system between the twoscrubbers.

It will be understood that in embodiments the sequence of stepsdisclosed may be modified, particular steps may be repeated one, two,three, four or more times, certain steps may be omitted and othermodifications may be made by those skilled in the art without departingfrom the scope and spirit of the subject matter hereof.

Mercury Scrubbers.

It is believed that the adsorption of mercury atoms at the gas solutioninterface may be an important step in the reaction process because itpermits the two reactions necessary for the removal of the mercury fromthe system as the insoluble product mercurous chloride (calomel), namelythe reaction of molecular chlorine with Hg, followed by the reaction ofnative Hg with HgCl₂ to form Hg₂Cl₂. The constant removal of mercuricions from solution by the formation of calomel may be offset by theoxidation of elemental mercury at the gas solution interface bydissolved chlorine gas. mercury scrubber (I), downstream of the SO₂scrubber, is used as both a reactor and a scrubber in the process.

Chlorine gas is injected into the scrubber liquid to both oxidize theHg⁰ to Hg⁺² at the gas/solution interface and to provide the chlorideion needed for the Hg⁺² to form soluble HgCl₂. Soda ash (SodiumCarbonate) is also added to the mercury scrubber as needed to controlpH. In embodiments it has been found that a pH of around 6.9 may besuitable to promote the reaction of molecular chorine with mercury. Thereaction between mercury and chlorine in mercury scrubber (I) thusrequires a different set point for pH than either the SO₂ scrubber ormercury scrubber (II) where the predominant reaction is that betweenHg⁰+Hg²⁺->Hg₂Cl₂.

Process Optimization

The rate of mercury removal in this system is shown by the followingequation:

−dC _(Hg) /dt=K·A·f(p)·f(T)

where: p=partial pressure of mercury in the off gas

-   -   K=a constant    -   A=the surface area of the solution    -   T=the temperature of the gas liquid interface

At any one temperature the adsorption of mercury atoms at the gassolution interface may be governed by a form of the Langmuir adsorptionisotherm which in its simplest form approximates to:

θ=α·p/(1+α·p)

where: p=partial pressure of mercury in the off gas

-   -   α=a constant    -   θ=fractional coverage of the surface

It will be apparent that the following parameters may significantlyaffect the reaction: the pH of the solution; the ORP value of thesolution; the concentration of mercuric ions in the solution; theconcentration of chlorine in solution; the concentration of sulfurdioxide in solution; and the concentration of chloride in solution.

In a general embodiment there are provided two separate scrubberscarrying out two different functions. Each has different optimizationparameters.

In the first scrubber, mercury scrubber (I), both Reaction A(Hg⁰+Cl₂->HgCl₂) and Reaction B (Hg⁰+HgCl₂->Hg₂Cl₂) occur. BothReactions A and B will control the mercury concentration in the gas andwill assist in keeping the level of mercuric ions where they need to be,in the solution, to provide the measure of control to produce thedesired reduction in the mercury vapor pressure in the gas.

In mercury scrubber (I) the parameters to maximize Reaction (A):

Hg⁰+Cl₂->HgCl₂ are more critical

In the second scrubber, mercury scrubber (II), the Reaction (B):

Hg⁰+HgCl₂->Hg₂Cl₂

is promoted, the creation of Calomel by the oxidation of the remainingmercury vapor with mercuric chloride to produce insoluble mercurouschloride (calomel). Since the primary objective of mercury scrubber (II)is to convert elemental insoluble mercury into its oxidized insolubleform (Hg₂Cl₂), removing it from the gas stream, the scrubber'sperformance is increased when the concentration of the mercuric chloridein the bulk solution is sufficient to create an effective concentrationof atomic mercury, across the boundary layer, of zero.

ALTERNATIVE EMBODIMENTS

In a second series of embodiments there is disclosed a method forcleaning a gas, which may be a flue gas from ore roasting or otherprocesses. The method may comprise removing mercury from the gas, andthis may be achieved by oxidising mercury with a solution of chlorinegas. In an alternative embodiment of the second embodiment, there isdisclosed a method for removing mercury from a gas wherein theimprovement comprises reacting said mercury with chlorine gas to formHgCl₂. In alternative embodiments the chlorine gas may be in aqueoussolution and the method may further comprise reacting said mercury withsaid HgCl₂ to form Hg₂Cl₂. In a further embodiment there is disclosed amethod for removing mercury from a gas wherein the improvement comprisesreacting said mercury with dissolved chlorine gas, which reacting mayoccur at a gas liquid interface.

In a further series of embodiments there is disclosed an apparatus forimplementing the other embodiments. The apparatus may comprise a firstmercury scrubber for reacting said mercury with a solution of chlorinegas to generate HgCl₂. In further embodiment the apparatus may furthercomprise a second mercury scrubber for reacting the mercury with asolution of HgCl₂ and generating Hg₂Cl₂. In a further embodiment theapparatus may comprise a precipitator or harvester to removeprecipitated Hg₂Cl₂.

In a further embodiment there is disclosed a mercury scrubber whereinthe improvement comprises a reactor for reacting the gas to be treatedwith a chlorine solution and wherein a further improvement comprisesprecipitating mercurous chloride.

In an embodiment there is disclosed a method for removing mercury from agas, the method comprising reacting the mercury with chlorine gas toform mercuric chloride. In alternative embodiments the chlorine gas andthe formed mercuric chloride are in aqueous solution. In embodiments thesolution and the gas may form a gas/liquid interface and the reactingmay further comprise forming a 2-dimensional mercury vapour at saidgas/liquid interface. In embodiments the reacting occurs at atemperature of less than about 100° C. In embodiment the method furthercomprises the step of reacting any unreacted mercury with the mercuricchloride to form mercurous chloride and may comprise the step ofremoving the mercurous chloride from the solution. In embodiments thechlorine solution is at a pH of between about 6.5 and about 7.5 and inembodiments the gas may be an off gas.

In an embodiment there is disclosed an apparatus for removing mercuryfrom a gas, wherein the improvement comprises a first mercury scrubberfor reacting the mercury with a chlorine solution. In embodiments theapparatus may comprise a chlorine feed system for regenerating thechlorine solution. In embodiments the improvement further comprises asecond mercury scrubber and a circulation system suitable tosubstantially continuously recirculate the chlorine solution between thetwo said scrubbers. In embodiments the apparatus further comprises aharvester for harvesting precipitated mercurous chloride generated inthe scrubber.

In embodiments there is disclosed a method for removing mercury from agas wherein the improvement comprises reacting the mercury with asolution of chlorine gas to form mercuric chloride in solution. Inembodiments the method comprises reacting the unreacted mercury with themercuric chloride to form mercurous chloride. In embodiments the methodfurther comprises the step of removing the mercurous chloride from thesolution. In embodiments the solution and gas form a gas/liquidinterface and said reacting further comprises forming a 2-dimensionalmercury vapour at said gas/liquid interface. In embodiments the reactionof mercury with said solution of chlorine gas occurs in a first scrubberand said reaction of mercury with mercuric chloride occurs in a secondscrubber. In embodiments the method is carried out at a temperature ofless than about 65C or less than about 40C. In embodiments the reactionbetween the chlorine and the mercury occurs under an ORP of betweenabout 500 mV and about 2000 mV.

More generally, there is disclosed a method for removing mercury in offgases, in which the adsorbed mercury on the surface of the scrubbersolution reacts with the oxidizing agent in the scrubber solution and isabsorbed into the solution where the presence of the same oxidizingagent prevents it from being reduced into a elemental mercury by theaction of SO₂ etc. and scattered back into the off gases. The inventorhas identified a method of optimising the oxidation-reduction potential(ORP) in the scrubber solution by the injection of chlorine into thescrubber solution.

The present disclosure also provides a method for removing mercury inoff gases, in which mercury in exhaust gas discharged from roaster andother heat producing equipment is removed, characterized by including amercury oxidation process in which mercury in the off gases is adsorbedonto the surface of a scrubber solution and converted to mercurychloride by the reaction with dissolved chlorine gas in the solution.The mercuric chloride is then absorbed into the solution freeing thesurface to adsorb further mercury.

EXAMPLES

The following example is provided as illustrative of the firstembodiment and is not limiting. In alternative embodiments one or moreof the steps described may be omitted, modified or repeated orimplemented in a modified sequence, all in ways that will be readilyunderstood an applied by those skilled in the art to suit particularrequirements and circumstances. It will be understood that thetemperatures, and percentages presented in the example may be modifiedin particular embodiments. Alternative temperatures and temperatureranges will be readily recognised and implemented by those skilled inthe art.

In the example described, off gas produced by the roasting of gold oreis treated to remove mercury which may be elemental mercury.

1.1 General Gas Handling Process

A gas handling circuit pulls the off-gas from the source ore roaster andprocesses it to remove dust particulate material, sulfur dioxide gas.other gases and mercury vapor before releasing clean gas to theatmosphere through the stack. A negative differential pressure withrespect to the atmosphere may be maintained inside the system to preventdust and gases from escaping the system. The differential pressure maybe controlled by the operation of the gas handling circuit.

Hot dust-laden off-gas from the system is directed to the inlet of a gasquench tower, where it may be sprayed with water. At this point it maybe cooled from its starting temperature which may be or may be above orbelow about 620° Centigrade (C.) to a lower suitable temperature whichmay be about 55° C. or higher or lower, by heat loss to waterevaporation and slurry heating, prior to exiting the gas quench tower.This cooling step may reduce the total gas volume and may do so byapproximately 60%. The majority of the particulate matter larger than 2microns in diameter may be captured in this water and leaves the quenchtower, as dilute slurry. The size cut off may vary depending on detailedprocess parameters. The cooled gas stream then enters the dust scrubberwhere it may be sprayed with water. The bulk of the remaining largerparticles are captured in this fluid stream flowing through the dustscrubber.

The cooled gas stream then passes upwards and out of the dust scrubberto the sulfur dioxide scrubber. Further heat transfer in the dustscrubber may cause the gas temperature to drop to approximately 35° C.

Following the dust scrubber, the gas may be then passed through a packedbed soda ash scrubber (the sulfur dioxide scrubber) to remove any sulfurdioxide that may be present. Next a single stage induced draft fanboosts the scrubbed gas pressure by about 3.5 pound per square inch(psi) for subsequent transfer to the mercury scrubbers (I) and (II). Themercury scrubber (I) contacts the gas with a dilute solution of soda ashfor pH control, this solution also contains both dissolved chlorine gasand mercuric chloride for mercury oxidation and control. The gas streamthen passes through the mercury scrubber (II) which contacts the gaswith a solution of soda ash and mercuric chloride for further mercuryrecovery and control. The gas stream then passes to the tail gasscrubber as a final cleaning, and may be discharged to the atmospherethrough the stack.

1.2 Gas Quench Tower

Hot gas (620° C.) from the system may be routed through a stainlesssteel duct to the inlet at the top of the gas quench tower. A gas inletnozzle projects 2″ through the lid to maintain a sharp temperaturetransition into the tower, which minimizes corrosion and solids buildupon the inlet nozzle. As the hot gas passes through the inlet nozzle,four water spray nozzles quench the gas stream. These nozzle assembliesare located at 90° angles just below the inlet. Each 6″ nozzle projectsa flat spray perpendicular to the direction of the gas stream.Consequently, the gas stream may be completely surrounded by a wall ofwater, which penetrates the gas stream to its core.

There are 8 additional spray nozzles in the upper section of the gasquench tower that spray onto the vessel walls to minimize corrosion andsolids deposition. The hot gas evaporates water and may be cooled toabout 75° C. in the process. As the gas passes through the water sprays,the gas quench tower vessel diameter increases. Both the cooling andincreased diameter serve to slow down the gas from the inlet.

Another water nozzle located near the bottom of the gas quench towerprojects a water spray upward against the gas flow. The gas leaving thequench tower may be saturated with water at 55° C. and contains somewater mist. Slurry discharging from the quench tower passes through a 3″drainpipe to a tank located directly below the quench tower.

1.3 Wet Dust Scrubber

The wet dust scrubber may be located next to the gas quench tower andmay be connected to it with a short section of rectangular duct.Saturated gas from the gas quench tower, laden with mist, dust andsulfur dioxide, enters the scrubber tangentially near the bottom. Theinner wall of the lower section may be protected from abrasion with aceramic shield. Gas may be forced through the stationary vane cage inthe lower section, where it may be contacted with water from thehigh-pressure water line. The water spray may be broken into tinydroplets by the action of the swirling gas as it passes through thevanes of the cage. The water droplets capture dust particles as the gaspasses up through the scrubber. The resulting slurry may be drained fromthe bottom of the Entoleter scrubber to the seal tank. The gas and mistpass up into the disengaging section of the Entoleter where the waterdroplets are collected on the vessel walls. The cleaned gas passes outof the scrubber through the top flange and then travels down through theFRP duct (fiberglass reinforced plastic) to the sulfur dioxide scrubber.

Efficient scrubbing in the scrubber increases with increasing pressuredrop. The pressure drop may be controlled by the vane cage height and,to a lesser extent, by the water flow rate. The vane cage height can beadjusted by means of an adjusting ram located in the side of the unit.

1.4 Sulfur Dioxide Scrubber

The Sulfur Dioxide scrubber uses a dilute (10%) soda ash solution toscrub sulfur dioxide from the gases leaving the wet dust scrubber. Thegases pass up through a packed bed where they are contactedcountercurrent with the dilute soda ash solution. Soda ash makeup to thescrubber sump may be controlled by the pH of the sump. The scrubbedgases pass through a Chevron-type mist eliminator before leaving thescrubber. The gasses move on to the induced draft fan, and the spentscrubber solution may be bled from the sump recycle line to control thesump level. Spent scrubber solution may be sent to either the effluenttank.

The recirculating soda ash solution may be cooled as it passes through aheat exchanger using cold water from the tailings seepage as the coolingmedium. The intent may be to keep the temperature of the soda ashsolution entering the scrubber at approximately 30° C.

1.5 Induced Draft Fan

The sulfur dioxide scrubber gas discharge feeds the induced draft fansuction and the fan discharges approximately 5,000 scfm of waste gas tothe mercury scrubber. The system first stage pressure indicatingcontroller adjusts the suction damper to the induced draft fan in orderto control pressure inside the system.

Gas feed to the induced draft fan discharges from the sulfur dioxidescrubber. This waste gas may be low in SO₂, but may contain somesulfuric acid mist. At normal operating conditions the gas may be about30° C. at the induced draft fan suction damper. Waste gas may bedischarged from the fan at about 35° C.

1.6 Mercury Scrubbing Systems (I) and (II)

Elemental mercury in the waste gas from the induced draft fan may bescrubbed out in the mercury scrubbers (I) and (II) and the cleaned gasmay be sent on to the tails gas scrubber. Waste gas from the roastercontains mercury vapor derived from mercuric compounds in the ore thatoxidize within the roaster. Some of the mercury vapors are condensed inthe SO₂ scrubber and the mercury scrubbers (I) and (II) are designed tocapture the remainder.

The waste gas may be contacted in mercury scrubbers (I) and (II)countercurrent with a solution containing both dissolved chlorine gasand mercuric chloride in the 20 foot packed beds. The pH of the scrubbersolutions may be controlled by addition of soda ash. A bleed stream fromthe mercury scrubbers' recirculation tanks, which contains both mercuricchloride in solution and a slurry of calomel, may be combined with ableed from the sulfur dioxide scrubber and the Tails Gas scrubber. Thiscombined bleed stream may be then treated for soluble mercury content bythe addition of zinc dust to produce mercurous chloride precipitate(calomel) the precipitate may be collected and the solution sent to theeffluent tank and on to tails. An automated chlorination system feedschlorine into the line prior to the mercury scrubber (I). This chlorinereacts with the elemental mercury vapor thereby providing the mercuricions for the mercury scrubbers (I) and (II) that may be lost to thecalomel precipitate. ORP of the scrubber solution is generallymaintained at approximately 900 mv or higher and mercuric chloridelevels are generally maintained at levels of between about 500 ppm to2000 ppm

The mercury scrubber (I) solution recycle stream may be taken from the4″ nozzle on its sump and sent to a header that feeds 3 recycle pumps.The inside pump may be the primary recycle pump for the mercury scrubber(I). The outside pump may be the primary recycle pump for mercuryscrubber (II). The middle pump may be a spare, which can serve eitherthe mercury scrubber (I) or (II). Each pump operates at 300 gpm. The 4″line from the pump discharge recycles solution from mercury scrubber (I)to one of the spray heads at the top of the tower of mercury scrubber(II). The sump level controller, of mercury scrubber (I) operates thecontrol valve on the 1″ line from the pump discharge, that discharges tothe sump of mercury scrubber II, to maintain the sump level. Two pHprobes and one ORP probe are in the 4″ recycle line and control theaddition of soda ash and chlorine gas to the system respectively.

The mercury scrubber (II) solution recycle stream may be also taken froma 4″ nozzle on the sump and sent to the same header that feeds 3 recyclepumps. The inside pump may be the primary recycle pump for the mercuryscrubber (I). The outside pump may be the primary recycle pump formercury scrubber (II). The middle pump may be a spare, which can serveeither the mercury scrubber (I) or (II). Each pump operates at 300 gpm.The 4″ line from the pump discharge recycles solution from mercuryscrubber (II) to one of the spray heads at the top of the tower ofmercury scrubber (I). The sump level controller, of mercury scrubber(II) operates the bleed control valve on the 1″ line from the pumpdischarge, which discharges to the bleed collection tank, to maintainthe sump level.

Chlorine gas may be injected into the sump of mercury scrubber (II) tobe fed to the spray nozzles at the top of mercury scrubber (I). Theamount of chlorine gas injected to the sump of mercury scrubber (II) tobe fed to the spray nozzles at the top of mercury scrubber (I) may becontrolled by an automatic valve and the flow rate adjusted by thereading of the ORP probe in the feed line to the mercury scrubber (II)spray nozzles.

Soda ash may be fed to the mercury scrubber sumps from a 10% soda ashmakeup tank. The soda ash makeup rate may be controlled by the pHcontroller set at pH 8.0. Waste gas may be discharged from the mercuryscrubbers at 85° F./30° C.

1.7 Tails Gas Scrubbing System

The tails gas scrubber acts as a final polishing device. The tail gasscrubber may be a 25 foot tall packed tower, 6 foot in diameter withstainless steel walls. The tower has a 600-gallon sump, and 8″Chevron-type 4-pass mist eliminator with a water spray, and one sprayhead for solution recycle. The tower may be rated at 6 psig pressure and100 inches of water column vacuum. Water may be piped to the misteliminator spray on the tail gas scrubber. The flow rate may becontrolled by a ball valve and may be monitored locally. Soda ash may befed to the tail gas scrubber sump under the control of the pHcontroller. A high pH alarm may be included in this controller to warnof high pH conditions, which can result in plugging of the packed beddue to sodium bicarbonate scale formation on the packing.

The embodiments and examples presented herein are illustrative of thegeneral nature of the subject matter claimed and are not limiting. Itwill be understood by those skilled in the art how these embodiments canbe readily modified and/or adapted for various applications and invarious ways without departing from the spirit and scope of the subjectmatter claimed. The claims hereof are to be understood to includewithout limitation all alternative embodiments and equivalents of thesubject matter hereof. Phrases, words and terms employed herein areillustrative and are not limiting. Where permissible by law, allreferences cited herein are incorporated by reference in their entirety.It will be appreciated that any aspects of the different embodimentsdisclosed herein may be combined in a range of possible alternativeembodiments, and alternative combinations of features, all of whichvaried combinations of features are to be understood to form a part ofthe subject matter claimed. Particular embodiments may alternativelycomprise or consist of or exclude any one or more of the elementsdisclosed.

1. A method for removing mercury from a gas, the method comprisingreacting said mercury with chlorine gas to form mercuric chloride. 2.The method according to claim 1 wherein said chlorine gas and saidformed mercuric chloride are in aqueous solution.
 3. The methodaccording to claim 2 wherein said solution and said gas form agas/liquid interface and said reacting further comprises forming a2-dimensional mercury vapour at said gas/liquid interface.
 4. The methodaccording to claim 1 wherein the reacting occurs at a temperature ofless than about 65° C.
 5. The method according to claim 2 furthercomprising the step of reacting any unreacted mercury with the mercuricchloride to form mercurous chloride.
 6. The method according to claim 5wherein said method further comprises the step of removing the mercurouschloride from the solution.
 7. The method according to claim 2 whereinthe chlorine solution is at a pH of between about 6.5 and about 7.5. 8.The method according to claim 1 wherein the gas is an off gas.
 9. Anapparatus for removing mercury from a gas, wherein the improvementcomprises a first mercury scrubber for reacting the mercury with achlorine solution.
 10. The apparatus according to claim 9 furthercomprising a chlorine feed system for regenerating the chlorinesolution.
 11. The apparatus according to claim 9 wherein the improvementfurther comprises a second mercury scrubber and a circulation systemsuitable to substantially continuously recirculate the chlorine solutionbetween the two said scrubbers.
 12. The apparatus according to claim 9further comprising a harvester for harvesting precipitated mercurouschloride generated in the scrubber.
 13. A method for removing mercuryfrom a gas wherein the improvement comprises reacting the mercury with asolution of chlorine gas to form mercuric chloride in solution.
 14. Themethod according to claim 13 further comprising reacting the unreactedmercury with the mercuric chloride to form mercurous chloride.
 15. Themethod according to claim 14 wherein said method further comprises thestep of removing the mercurous chloride from the solution.
 16. Themethod according to claim 13 wherein the solution and gas form agas/liquid interface and said reacting further comprises forming a2-dimensional mercury vapour at said gas/liquid interface.
 17. Themethod according to claim 14 wherein said reaction of mercury with saidsolution of chlorine gas occurs in a first scrubber and said reaction ofmercury with mercuric chloride occurs in a second scrubber.
 18. Themethod according to claim 13 wherein said method is carried out at atemperature of less than about 65C.
 19. The method according to claim 13wherein said reaction between said chlorine and said mercury occursunder an ORP of between about 500 mV and about 2000 mV.